Comparison of methods to estimate population densities of black-tailed prairie dogs

Abstract

Recent reintroduction of the black-footed ferret (Mustela nigripes) in west-central South Dakota has focused new attention on black-tailed prairie dogs (Cynomys ludovicianus), because prairie dog colonies provide essential habitat for ferrets. Currently, management agencies are assessing prairie dog populations by counting active burrows, a technique that is attracting criticism. We correlated active and total burrow counts with prairie dog mark-recapture population estimates from 12 colonies located in Badlands National Park and adjacent Buffalo Gap National Grassland. We also correlated visual counts of prairie dogs and counts of mounds from aerial photographs with mark-recapture estimates to assess an alternative method to index populations. We found no significant relationships (P > 0.05) with any form of active burrow or total burrow counts (ground or aerial) using the linear model Y = a + bX. However, visual counts of prairie dogs, using maximum rather than mean values, on 4-ha plots were significantly related (P < 0.0138). The best model was Y = 3.04 + 0.40X, where Y is the maximum visual count and X is the estimated population density. The inverse of this equation X = (Y 3.04)/(0.40), could be used to index numbers of black-tailed prairie dogs from visual counts under conditions similar to those encountered in this study. An 8-point protocol for making visual counts is provided. burrow counts, Cynomys ludovicianus, mark-recapture, Mustela nigripes, population density, South Dakota, visual count Prairie dog (Cynomys spp.) colonies provide essential habitat for the endangered black-footed ferret (Mustela nigripes). Currently found only in captivity and on experimental release sites in Wyoming, South Dakota, and Montana, black-footed ferrets prey almost exclusively on prairie dogs and live in prairie dog burrows (Hillman 1968). Reestablishing viable populations of black-footed ferrets, therefore, depends on establishing and maintaining adequate numbers of prairie dogs within properly distributed colonies of optimum size (Houston et al. 1986). The recent black-footed ferret reintroduction into Conata Basin in west-central South Dakota has prompted managers of 2 agencies (U.S. Dep. Agric. For. Serv. [Buffalo Gap Natl. Grassland] and Natl. Park Serv. [Badlands Natl. Park]) to reassess methods of d termining sizes of populations of black-tailed prairie dogs (C. ludovicianus). Most agencies rely on a protocol developed by Biggins et al. (1993) to assess black-footed ferret habitat. The protocol is based on population estimates derived from counting numbers of active prairie dog burrows. Justification for this was a set of unpublished data that reported fair and good relationships between counts of active burrows and blackand white-tailed prairie dogs (C. leucurus), respectively (Biggins et al. 1993). Powell et al. (1994) suggested that counts of active burrows may not be a reliable indicator of black-tailed prairie dog populations. Menkens et al. (1988) examined relationships between populations estimated by Address for Kieth E. Severson: USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, School of Mines Campus, Rapid City, SD 57701, USA. Address for Glenn E. Plumb: National Park Service, Badlands National Park, Interior, SD 57750, USA. Wildlife Society Bulletin 1998, 26(4):859-866 Key words